This invention relates generally to suspension systems for hanging objects and more particularly to suspension system for hanging a susceptor adapted to hold semiconductor wafers inside a reactor, such as an epitaxial reactor, during semiconductor processing.
FIG. 1 schematically illustrates a suspension system 20 for suspending a susceptor 26 inside a reaction chamber 23 of an epitaxial reactor 22. Reactors of this type are commercially available from Applied Materials, Inc. of Santa Clara, Calif., U.S.A. as models AMC-7810/11 and AMC-7820/21 Cylindrical Epitaxial Reactors. In reactor 22 the reaction chamber 23 is defined within an inverted quartz bell jar 24 made of a semiconductor-grade clear-fused quartz which is substantially transparent to infrared radiation, allowing for high heat transmission. The quartz is generally at least 90% pure silica and is substantially chemically inert to the environment within the reactor.
Susceptor 26 has a number of flat faces provided with hollow pouches which support semiconductor wafers 28. The number of flat faces is primarily determined by wafer size. A typical susceptor is a single-piece structure having its sides sloping outwardly from top to bottom to permit the flow of reactant gases across all the wafers and to prevent wafers from being dislodged from the pockets. Banks of suitable heat sources such as quartz-halogen lamps 30 are arrayed around the exterior of the bell jar and are powered by a lamp power supply for radiantly heating both susceptor 26 and wafers 28.
The lamp module design of an Applied Materials model AMC-7820/21 epitaxial reactor includes several lamp modules containing twenty 6000 watt quartz-halogen lamps generating infrared (IR) radiant light. Each lamp is associated with a gold-plated aluminum parabolic reflector to more efficiently direct the IR radiation towards the susceptor. Gases for epitaxial processing of the wafers are supplied through gas inlet ports, while radiation product waste gases are removed through exhaust ports.
FIGS. 2 and 3 show details of suspension system 20, which includes a quartz hanger 32 having a lower end 34 provided with a flange 36. Susceptor 26 of FIG. 1 is fitted at its bottom and top respectively with a removable lower cover 38 and upper cover 40, the latter defining through itself a cover aperture 42. A quartz tab 74 is joined to flange 36 and to the side of hanger 32; tab 74 engages a notch (not shown) in upper cover 40 of the susceptor to correctly align the collar and susceptor with respect to each other within the reactor. To suspend the susceptor, hanger 32 is mounted inside aperture 42 so flange 36 is in contact with the lower side of upper cover 40 to thereby support susceptor 26. An upper end 44 of hanger 32 is secured to an anchor 46 (FIG. 1) of a susceptor support mechanism formed in a removable reactor top plate assembly 48 using appropriate pressure seals.
Top plate 48 is coupled to drive assembly (not shown) which can lift and lower the suspension system and susceptor within the reaction chamber, and which can rotate the suspension system and susceptor during wafer processing to increase deposition uniformity. An instrument probe conduit 50 extends through top plate 48, anchor 46, hanger 32 and into the hollow interior 52 of susceptor 26 so that sensors such as a temperature probe 54 can monitor the operating parameters within the reactor.
Prior art suspension system 20, shown in exploded form in FIG. 2 and partially assembled in FIG. 3, further includes a collar 70 and a retainer 72, the latter two preferably manufactured from 316H or 174-PH stainless steel due to their high corrosion resistance. The hanger, collar and retainer each has a circular cross-section and are substantially coaxial about a common axis 73. Hanger 32 includes a first barrel 76 with an outside diameter OD1 and a second barrel 78 with an outside diameter OD2, separated by a relatively thin circular flat beveled face 80. The hanger has an inside diameter ID1 which is substantially uniform throughout the length of quartz hanger 32.
The OD1, OD2 and ID1 dimensions have been optimized relative to each other and with respect to the reactor parts with which hanger 32 interacts. Among the factors affecting this optimization are the mechanical strength of quartz, the diameter of aperture 42 in upper cover 40 of susceptor 26, the complex structure (not shown or discussed here) of top plate 48 including a gas flow path for admitting reactor gases along the surface of OD2 and into the reactor, the size and structure of anchor 46, and the size of instruments such as temperature probe 54 that must fit through probe conduit 50 for gaining access to hollow interior 52 of susceptor 26.
Toward upper end 44 of hanger 32, holes 82, 83, 84 and 85 are provided through the wall of the hanger. Three holes 82, 83 and 84 are spaced apart 90 degrees; hole 85 is 105 degrees away from hole 84 and 75 degrees away from hole 82 to provide proper alignment of suspension system 20 within the reactor. Collar 70 has four holes 86, 87, 88 and 89 positioned to be alignable respectively with hanger holes 82, 83, 84 and 85.
Retainer 72 has a lower edge 92 into which semicircular notches 94 96, 98 and 100 are formed in the same spaced apart orientation respectively as collar holes 86, 87, 88 and 89. When hanger 32, collar 70 and retainer 72 are properly oriented for assembly, hanger holes 82, 83 and 84 and collar holes 86, 87 and 88 line up respectively with retainer notches 94, 96 and 98, while offset hanger hole 85 and offset collar hole 89 are aligned with offset notch 100. To complete this assembly, four rivets 102, 104, 106 and 108, also preferably manufactured from 316L or 174-PH stainless steel, are manually inserted through the holes and notches to join together collar 70 and retainer 72 for supporting hanger 32 directly from the four rivets. Four J-shaped slots 110, 112, 114 and 116 are cut 90 degrees apart into an upper collar rim 118 to mate with connectors (not shown) provided in anchor 46.
Even though the suspension system shown in FIGS. 2 and 3 has been used successfully for a number of years, certain disadvantages have come to be identified with this construction including the difficulty of its manufacture and assembly and its occasional premature failure due to fracturing of the hanger.
During manufacture, quartz hanger 32 is formed as a cylindrical tube that is then machined on a lathe to cut second barrel 78 and beveled face 80. Lathe machining provides precise dimensioning of the hanger exterior surface. Face 80 is cut at an inclined angle rather than a 90 degree angle to maintain the mechanical strength of the quartz hanger and for ease of manufacture when advancing the lathe cutting tool (not shown) inwardly from barrel 76 to cut into the hanger for forming barrel 78. A boring tool is used to cut holes 82, 83, 84 and 85 through hanger 32, and the holes are flame polished to remove rough edges. Unfortunately, drilling and flame polishing often produces holes having imprecise shapes and dimensions, causing alignment problems between the holes of hanger 32 with the corresponding openings machined into collar 70 and retainer 72. Such misalignments can cause hanger 32 to hang off-center from the rivets so the susceptor is not properly centered for optimal processing of the wafers and can create unbalanced stresses on the quartz hanger. Furthermore, drilling and heat treating can weaken the structure of quartz hanger 32.
Undesirable wear results from the above misalignment, and also from the load-supporting contact between the steel rivets and the quartz hanger. Because steel and quartz have different coefficients of thermal expansion, the thermal cycling inherent with the use of reactor causes the steel and quartz to expand and contract at different rates, resulting in the development of small cracks at the circumference of the holes and causing the quartz to wear away and flake off. These effects both weaken the quartz hanger and introduce particulate contamination into the reactor that can damage the wafers carried by the susceptor. In some cases the weakened holes in the quartz hanger can cause the end of the hanger to break off, allowing the susceptor and wafers to crash to the bottom of the bell jar.
The prior art suspension system is also difficult to assemble. Hanger 32 must be supported while collar 70 and retainer 72 are slipped over the end of the hanger and then appropriately aligned to permit insertion of the four rivets. Such multiple alignments are difficult to achieve while at the same time avoiding dropping any of the many pieces.